The proposed investigation is a two-phase project designed to: (a) identify the best combination of physiological and psychological factors which explain why some children show extreme reactivity to psychological stress, and to (b) test whether aerobic exercise training can modulate such extreme reactivity. In Phase 1 of this project, two groups of normotensive adolescent males (30 subjects with a parental history of hypertension and 30 subjects without) will be subjected to a psychologically stressful laboratory protocol for the purpose of identifying the physiological and psychological traits which characterize those who exhibit extreme cardiovascular and neuroendocrine reactivity to the protocol. 'Re identified personal traits could include: parental history of hypertension, low level of cardiovascular fitness, high trait anxiety, external locus of control, high levels of hostility and anger, and/or low self-concept. In Phase 2 of this project, 40 adolescent boys who possess the physiological and psychological traits identified in Phase 1 as those which are associated with hyperreactivity to psychological stress will be selected and randomly assigned to one of two treatment groups. The 20 subjects in Group 1 will undergo a 14-week program of aerobic exercise training while the 20 subjects in Group 2 will be assigned to an attention-placebo control group. Following the 14-week training period, the two groups of subjects will again be subjected to the stressful laboratory protocol to determine if aerobic exercise training can modulate hyperreactivity to psychological stress. Given the body of evidence linking psychological stress reactivity to the pathogenesis of cardiovascular disease conditions, the practical significance of these studies lies in their ability to identify the physiological and psychological traits of children who may be at risk for heart disease and then to investigate whether a non-pharmacological intervention technique (e.g., exercise training) has the potential to reduce the vulnerability of such disease processes. In addition to the practical significance of this project, we also intend to examine issues of more theoretical importance. Specifically, we propose to further explore the hemodynamic and neuroendocrine changes that occur during psychological stress. Theoretically, the metabolically exaggerated cardiovascular responses to psychological stimuli need further clarification with respect to the pathogenesis of hypertension. The data obtained through the exercise training portion of this study will be examined to test the hypothesized mechanisms by which aerobic training may modulate psychophysiological reactivity to cognitive stimuli. Thus, the proposed experiments will yield results that offer both practical and theoretical significance.